Electric motors, particularly induction motors with variable speed, are widely used due to low maintenance cost and acceptable performance. However, the control of induction motors is challenging due to highly coupled and nonlinear dynamics. Vector control, also called field-oriented control (FOC), is a variable-frequency drive (VFD) control method where the stator currents of a three-phase AC electric motor are identified as two orthogonal components that can be visualized with a space vector. One component defines the magnetic flux of the motor, the other the torque or speed. The control system of the motor drive calculates from the flux and speed or torque references given by a high level controller the corresponding current component references. For example, proportional-integral (PI) controllers can be used to keep the measured current components at their reference values.
Speed sensorless control of the electric motors avoids measuring the speed of the motor. Such a control can also be implemented by a speed sensorless motor drive, i.e., control system that does not use a sensor to measure speed or position of the rotor of the motor. Speed sensorless motor drives are desirable due to the elimination of motor speed or position sensors, the lower cost, and the improved reliability of the resultant system. For example, conventional speed sensorless control technologies include a voltage model-based direct integration, an adaptive observer method, and an extended Kalman filter method to estimate the rotor speed.
However, the speed sensorless motor drives can fail to control the motor effectively. Specifically, a number of different speed sensorless motor drives suffer from performance degradation resulting from removing the encoder (the sensor to measure the position of the rotor) from the control loop, i.e., a position sensor from which the rotor speed can be quickly obtained. For example, the existing speed sensorless control can fail to accurately and rapidly track a fast-changing speed reference trajectory of the electric motor. To that end, the application of the speed sensorless control technologies is limited to fields requiring low or medium performance of the motor.
Accordingly, there is a need to improve the performance of the speed sensorless control of the electric motors.